The invention pertains to semiconductor die and other device packaging. More particularly, the invention pertains to encapsulation of semiconductors and other devices by stencil printing.
Stencil printing was originally introduced to the semiconductor field for use in placing small formations, such as solder bumps, on the surfaces of semiconductor dies. Essentially, the semiconductor dies were placed under stencils or screens with apertures corresponding to the places on the surfaces of the die where, for example, solder bumps were to be placed. The depth or height dimension of the stencil is selected to be equal to the desired height of the solder bumps. A viscous solder paste is then applied over the stencil with a wiper or squeegee oriented at an acute angle to the top surface of the stencil. The squeegee traverses the stencil and pushes the solder paste ahead of it as well as down into the apertures, thus depositing the solder paste in the desired locations of the solder bumps on the surface of the semiconductor die. The stencil is then removed.
Some have attempted to use stencil printing in other applications pertaining to semiconductors. Particularly, stencil printing has been attempted for encapsulating semiconductor dies. A stencil printing process for encapsulation of semiconductor dies might involve placing a plurality of dies on a substrate in a regular rectangular pattern such that there are a plurality of parallel vertical streets and a plurality of parallel horizontal streets defining the spaces between the dies. It should be understood that the terms vertical and horizontal are arbitrary and are not intended to define any particular orientation of the streets to the horizon, but merely that the two sets of streets are more or less orthogonal to each other. The terms horizontal and vertical are used herein because they are the terms generally used by persons in the related arts. A stencil is then rested on the substrate so that the dies appear in the apertures of the stencil.
Each die may correspond to a separate aperture in the stencil, such that the streets are defined by the spaces between the apertures in the stencil. However, more likely, each aperture in the stencil will contain a plurality of dies laid out in a rectangular pattern. The stencil may have a plurality of such apertures also laid out in a rectangular pattern.
The encapsulant is applied into the apertures as described above using a squeegee that runs over the stencil and forces the epoxy into the apertures in the stencil, covering all sides of the dies therein, except for the side face down and in contact with the substrate. Depending of the particular process, the dies may be placed face up or face down on the substrate. In either event, the surface that is face down on the substrate does not get covered with the encapsulant. However, all of the other sides do. The substrate itself will act as the protective cover for the face down side of the die.
In semiconductor fabrication, the encapsulating material typically is a relatively high viscosity liquid epoxy that must be cured after the stencil printing in order to harden it into its final form. Accordingly, the stencil is then removed and the workpiece, comprising a substrate bearing a plurality of dies captured within uncured epoxy, is placed in a curing oven to heat cure and harden the encapsulating material. Typically, a plurality of these workpieces are cured simultaneously in the curing oven. The number of workpieces cured simultaneously depends on many factors, including the size of the oven, and may range anywhere from dozens to tens of thousands.
After curing, the workpiece is diced along the horizontal and vertical streets in order to separate the encapsulated semiconductor chips from each other.
In encapsulation applications, the apertures in the stencils obviously are much larger than in solder bumping applications The size of the apertures when stencil printing is used for encapsulation of semiconductor dies can range as high several inches across each side (assuming a square or rectangular aperture, for example). Rectangular apertures of two to six inches per side (e.g., 4 to 36 square inches) for accommodating 16 to 25 dies in each aperture is exemplary. Common sizes include 0.75xe2x80x3xc3x970.75xe2x80x3 and 2xe2x80x3xc3x972xe2x80x3. The stencil apertures should have a height equal to the height of the dies plus the desired depth of the encapsulant on the top surface of the dies. On the other hand, the apertures found in stencil printing for solder bumping typically will be generally circular and range in diameter from about 50 to 100 microns and be much less in height.
The viscosities of presently available encapsulating materials for semiconductor dies range from about 50,000 centipoise to about 1,000,000 centipoise. However, due to any number of circumstances, there may be a lengthy delay between removal of the stencil from the substrate/dies and final curing, during which time the viscous liquid epoxy encapsulating material can slump or flow. For instance, at many manufacturing facilities, the number of available stencils accommodates far fewer dies than can be cured simultaneously in the curing oven. Accordingly, the stencils are removed from a first set of dies and reused before the first set of dies are placed in the oven for curing. It would not be uncommon for the delay between removal of the dies from the stencil until curing to be several hours. Accordingly, semiconductor manufactures often must select the particular encapsulating material for a given application based, at least partially, on it having a sufficiently high viscosity to minimize slump during this delay period. This problem becomes more acute as the size of the workpiece increases (as the size of the apertures in the stencil increase) or as the height of the encapsulation increases.
Accordingly, if there might be any significant delay between removal of the stencil and final curing of the encapsulant, the semiconductor manufacturer likely will need to select an encapsulating material with a viscosity of at least about 300,000 centipoise in order to minimize slump.
Several stencil printing machine manufacturers now offer stencil printing machines for solder bumping in which the solder paste is contained in a pressurized vessel so that the solder paste is more forcibly injected into the apertures in the stencil. One such line of machines is the Horizon series of stencil printing machines manufactured by DEK, Inc. of Surrey, England, which includes, among others, the Horizon 265 model. In such stencil printing machines, the solder paste is contained in a pressurized vessel to pressurize the solder paste. At the bottom of the vessel is a printing head that includes a long, narrow slot with two wipers or squeegees, one on each longitudinal side of the slot. The slot and wipers ride over the stencil forcing the paste out of the slot into the apertures in the stencil.
It is an object of the present invention to provide an improved stencil printing apparatus.
It is another object of the present invention to provide an improved method for encapsulating semiconductor dies or other devices using stencil printing.
It is a further object of the present invention to provide an improved stencil printing apparatus for encapsulating semiconductor dies or other devices.
It is yet a further object of the present invention to provide an improved apparatus for encapsulating semiconductors using stencil printing.
In accordance with the present invention, semiconductor dies and other devices are encapsulated using a stencil printing technique in which, during and/or after the dies have been stencil printed with encapsulant and prior to removal of the stencil from the dies, the edges of the encapsulant are partially light cured in order to prevent or reduce slump of the encapsulant in the time period between removal of the stencil and curing of the encapsulant. In particular, the stencil is provided with a plurality of light pipes that terminate at the edge walls defining the apertures within which the dies are placed. Ultraviolet (UV) or other curing light is provided through the light pipes to the edges of the encapsulating material. The light cures just the edges of the encapsulating material so that the stencil can be removed without the encapsulating material slumping. This enables semiconductor chip manufacturers the flexibility to use lower viscosity encapsulating materials and/or to increase the delay period between removal of the encapsulated dies from the stencil and curing of the encapsulating material since the encapsulating material will not slump because the edges are partially cured.
The light pipes may comprise channels in the stencil body with optical fibers therein. While it is within the scope of the invention for the stencils to have self-contained light sources within the body of the stencil for providing the light into the light pipes, in more preferred embodiments, the opposite ends of the light pipes terminate on or near the outer edge of the stencil where they can be mated to an external light source, such as a UV light source.
In other embodiments, the pipes may comprise hollow channels with high-reflectivity coating on the walls thereof. In even further embodiments, the entire stencil may be fabricated of a material, such as acrylic or polyethylene terephthalate glycol (PETG), that is transparent to UV light, whereby the entire stencil essentially is a large light pipe. An opaque or reflective coating (the reflective side facing inwardly of the stencil) may be applied to all surfaces other than the vertical surfaces defining the apertures in order to prevent light from escaping from the stencil at surfaces other than the surfaces at the apertures.
Optionally, some general or localized heating can be provided to enhance or accelerate the light curing of the edges of the encapsulant, but without substantially curing the rest of the encapsulant.